As I understand things, airlines generally give discounts for round-trips that consist of a flight from A to B followed by a flight from B to A. How does this logic work for multi-city flights? As an example (but don't focus strictly on this example), what about a flight from A to B followed by a flight from C back to A? What logic is applied to determine whether to charge as separate one-way trips or as a discounted multi-city trip? Is the important part that the trip begin and end in the same space, or does there need to be an unbroken cycle?

I know I can prod travel search engines for specific examples, but if I understand the rules it can help in my search. Is this airline dependent? Does it matter whether the flights are domestic or international (I'm only concerned here about domestic)?

2 Answers
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What you're describing is called an "Open Jaw" ticket, and there's a very simple rule for determining if it applies or not.

Lets say you're flying A-B and then C-A.

There's 3 distances involved here - A-B, B-C (ie, the overland segment) and C-A. For a ticket to be a valid "open jaw", the distance B-C needs to be shorter than both A-B and C-A.

eg, SFO-STL (1735 miles) and ORD-SFO (1846 miles) is a valid open jaw as the distance between the two cities is 258 miles, which is less than both of the flown legs.

SFO-LAS (414 miles) and DEN-SFO (967 miles) is NOT a valid open jaw routing, as the distance between LAS and DEN is 628 miles, which is greater than one of the flown legs.

For US domestic flights, it often doesn't matter if it's a valid open jaw or not. Many domestic flights are sold on a "one-way" basis, so it doesn't matter if your flights are a valid open jaw or not - or even if they are purchased on the same ticket or not - they will cost the same as in effect you're just paying for 2 one-way fares. Some fares do specify that they are only available for return or open-jaw, and that's when these rules do come into play.

For international flights, open jaw can make a huge difference to fares, as most cheap fares are not available one-way, they are only available as part of a return or open jaw ticket.

Actually, these days a return (a to b, and later b to a) ticket most often just costs the total of the two one-ways. Discounts for return trips are rare, though some discount codes or special offers are valid only if you book both ways. What some people have discovered is that c to a to b, and later b to a to c, can be cheaper than a to b and back, even when the a to b legs are on the exact same flights. (This is most common when a and c are on different sides of the Canadian border - YYZ-YVR-SEA and back is cheaper than YYZ-YVR a lot of the time.) So you get yourself to c somehow, and then start your trip. If it's a trip you make often, you can arrange for the final a to c leg to be a long time later (technically you're having a long stopver in a, where you probably live) thus solving the "get yourself to c" problem.

What you describe, a to b followed by c to a, is called an open jaw. It matters on reward tickets but not so much on a revenue (ordinary) ticket. I have done it on all kinds of flights and never noticed a significant price difference, even transatlantically.

For most of the flights I do, an A-B return is much much less than the cost of singles A-B then B-A, so it's not universal! Some flexible tickets may be priced that way, but in many cases (and especially if you have a Saturday night away) the return can be a lot less than two singles
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GagravarrMay 8 '12 at 11:41